Program scanning apparatus



Sept 29, 1959 w. w. DEIGHTON 2,906,833

PROGRAM SCANNING APPARATUS Filed March 4, 1955 Fig.4

7 INVENTOR.

WILLlAM \MDEIGHTON m3 jug 440451.

AGENT United States Patent PROGRAM SCANNING APPARATUS William W. Deighton, Darby, Pa., assignor to Burroughs Corporation, Detroit, Mich., a corporation of Michigan Application March 4, 1955, Serial No. 492,247

14 Claims. (Cl. 200-105) This invention relates to program scanning apparatus and, more particularly, to electrically controlled switching devices of the type which are adapted to be operated at a relatively high speed in a step-by-step sequence in response to successively applied actuating impulses in order to scan one circuit after another in serial fashion, e.g., in computers, business machines and the like.

Step-type switch and counter mechanisms which provide voltage adjustments in predetermined increments by stepping a contact member from one voltage tap to another are well known. Some of the more obvious limitations which have tended in the past to prevent the attainment of extremely high operating speeds stem from the fact that the earlier type switches have for the most part derived the power for the stepping operation directly from a solenoid or from the stored energy of a spring which is cocked by a solenoid. Since the load determines the size of the spring, the more mass there is to be driven the stronger the spring that must be utilized for this purpose and therefore the larger and more powerful the solenoid must be to drive or cook the spring. Stronger solenoids mean higher inductances which in turn slows their operating speeds. One well-known solenoid operated ratchet stepping switch, for example, requires approximately one-half ampere to energize the solenoid in order to cock the spring. Devices incorporating this latter type of electromechanical operation have been relatively large and bulky due to the driving forces involved and the increased potentials necessarily used therewith. For these reasons among others the utilization of switches of this general type has been limited to equipment which can tolerate relatively large size and relatively high operating potentials.

It is an important object therefore of this invention to provide a simple and eflicient program scanning switch that avoids the foregoing limitations and which attains a relatively high operating speed.

It is another important object of the invention to provide a high speed electromagnetically controlled scanning switch mechanism which is adapted to be driven from a source of constant torque.

It is another object of the invention to provide a relatively high speed stepping switch in which the driving source is independent of the stepping control function.

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, however, may be best understood by a reference to the following description taken in connection with the accompanying drawing, in which:

Fig. 1 is an enlarged isometric view of the program scanning switch assembly partially broken away to expose certain internal operating mechanisms of the apparatus;

Fig. 2 is an internal fragmentary view of the assembly of Fig. 1 showing the escapement mechanism of the invention;

Fig. 3 is another internal fragmentary view of the assembly of Fig. 1 showing a portion of one of the electrical distributor mechanisms used in the invention; and,

Fig. 4 is an end view of the assembly illustrated in Fig. 1.

Referring now more particularly to the drawing and to the various figures thereof which will be discussed simultaneously, it is seen that the program scanning switch control and operating mechanism of the present invention comprises a plurality of subassemblies which are disposed between and supported by a pair of substantially flat parallel extending rectangular side frame members 10-10 so as to form a unitary assembly. The frame members are connected together but held in spaced apart relationship by means of the bolts 22, 24, 26 and 28 and their associated spacers 29. The assembly generally includes a drive gear 14, a frictional coupling 16, an escap ment mechanism 18, and a printed wiring distributor 20.

A solenoid 30 having a winding 32 energizable over the leads 34, secured to a distributor plate to be described later on, is mounted between the frames 10-10 on an L-shaped bracket 36. The bracket may be secured as shown to the inwardly facing surface of one of the side frames by any suitable means such as the bolts 38. The solenoid is mounted on the plate with its axis extending vertically, and it is provided with a stationary core 42, the upper end of which is shown projecting above the solenoid body. The solenoid may be fastened to the bracket by reducing the lower end of the core 42 and projecting the same through the plate and thereafter peening over the reduced end to provide a rigid connec tion thereto. The bracket 36 is provided with an integral upstanding projection 44 which rises alongside of the solenoid and terminates at its upper end above the solenoid in a pair of ears 46-46 forming a ledge therebetween which serves as a fulcrum. The armatureof the solenoid is indicated at 48 and is shown overlying the upper projecting end of the core. One end of the armature is pivotally supported on the ledge between the ears 46-46 and retained thereon by the provision of two oppositely disposed notches 50-50 which interengage the two'ears. The armature is yieldingly biased away from the core 42 by means of a coiled spring 52 secured between the pivoted end of the armature and a projecting stud 54 on the bracket extension 44.

The escapement mechanism referred to hereinabove and generally identified at 18 includes an escapement arm or pawl 56 which is pivoted intermediate its ends by means of a lateral projecting shoulder portion forming a bearing 58journaled to the inner end of a frame-supported pin 60 through a standoff bushing 62. The pawl 56 and the solenoid armature 48 are mounted for movement about their respective pivotal axes in the same plane and to couple the two together so that the pawl is rocked by the armature the nearest end of the pawl is bifurcated as at 64 to receive the free end of the armature. The opposite end of the escapement arm or pawl is provided with two integral oppositely disposed pallets 66 and 68 which are capable of alternately engaging the teeth of an escapement ratchet wheel 70 as the pawl is rocked about its axis. The escapement wheel 70 is shown as forming an integral part of a hollow elongated shaft 72 extending between the frame members 10-10 but not connected thereto. The assembly of the escapement wheel 70 and hollow shaft 72-is rotatably'mounted as a unit on a solid center stud shaft 73 which is journaled to the two side frames 10-10 by means 'of the bushings 77 and 79. The hollow shaft 72 terminates short of one of the frame members to provide space for the securement of the drive gear 14 on the solid center shaft for joint rotation therewith.

The frictional coupling generally indicated at 16 is utilized to transmit a constant turning force or torque from the drive gear 14 to therescapement wheel 70 and thence to the distributor 20, to be described later on. The friction drive coupling comprises a pair of substantially C- shaped clutch arms 75-75 which conform in general to the curvature of the shaft 72 and extend along opposite side portions thereof. Each clutch arm is provided with an inwardly facing flat surface 74. One end of each of the clutch arms is overlapped upon the end of the other arm and the overlapping ends are pivoted to the gear 14 by means of a laterally projecting pin 76. The opposite free ends of the clutch arms are yieldingly drawn toward one another by a spring 78 attached at its ends to the projections 80-80 on the clutch arms. The spring biases the arms toward an enlarged hub 82 forming an integral part of the escapement wheel 70 and brings the-flats 74- 74 of the arms into frictional engagement therewith. The two clutch arms thus surround and grip the hub 82 and provide selective rotationof the ratchet wheel 70 at desired intervals, as willv presently be explained. Since the ratchet wheel, enlarged hub and hollow shaft are all one piece, the unitary assembly is mounted for rotation on the; common inner shaft 73. By virtue of the fact that the clutch arms contact the enlarged hub only along the portion of each. one of the arms which is substantially fiat; the coeflicient of friction between the arms 75-75 and the hub 82, while. highenough to effectively couple the input driving force of the wheel 14 to the ratchet wheel 70, is not high enough to become a lubricating or heat disposal problem. It is highly desirable to use arhardened, plastic, such as nylon, for the clutch arms, the hub, the hollow shaft and the ratchet wheel. This lightens the mass of these parts and thereby avoids any undue accumulation of mass which conventional metal parts would introduce while at the same time lowering the lubrication requirement of the device due to the fact that many of the rotating parts have the characteristic of being substantially self-lubricating.

Preferably located on the side of the escapement wheel 70 opposite to the clutch 16 are one or: more distributor mechanisms 20.each of which in the illustrated embodiment of the invention comprises a fixed card bearing exposed wiring and terminals on opposite sides thereof and a rotatable terminal contacting wiper assembly for each side of the card. Each wiper assembly is identified at 86 and is provided with a pair of arcuately shaped arms 87 whichas shown in Fig. 3 are located on diametrically opposite sides of the shaft 72-and have their centers of curvature coinciding with the axis of the shaft. Each wiper assembly is carried on an annular member or collar 90 of any well-known dielectric material and secured thereto in any suitable manner as by eyelets or rivets 91. Each collar is secured to the shaft 72 for joint'rotation therewith such as being keyed thereto as shown in Fig. 3. All'the wiper assemblies rotate as one unit with the hollow shaft 72 and the ratchet wheel'70.

Each card of the distributor mechanisms comprises a fiat sheet-like member or plate 94 also of dielectric material onto both sides of which conductors 96 have been formed in a conventional manner, as by etching or electroforming. The conductors extend toward the wipers in a general radiating fashion and terminate under the wipers in spaced relation to one another. The extremity of each arm carries a contact button 88. Each pair of wiper arms is arranged so that the conductive contact buttons 88 may contact both sides of the same card or plate simultaneously. The various individual cards or plates 94-94 may be insulatingly separated along the central shaft by means of a collar or bushing 98 also of dielectric material. A large conductive area identified at 100 extends for approximately 180 around the axis of the shaft and servesas a common terminal for the conductors 96. In operation, as is evident in Fig. 3, one wiper arm makes contact with the common terminal 100-in any co'ntrasting position to which the other wiper arm may be adjusted with respect'to the conductors 96.- Each common conductive area may thus be successively connected to the individual conductors 96.

In order to improve the efiiciency of the mechanism and to utilize the pluggable feature thereof to its fullest extent, the two leads 34, for the solenoid 30, are electrically connected to two separate conductors 96 of the distributor. This may be accomplished as shown in Fig. 1 by connecting the leads to two corresponding conductors 96 on opposite sides of one of the distributor cards 94 by means of eyelets 97 'and 99, mounted in the card.

Each of the conductive areason the distributor cards are connectible to a separate external circuit. For this purpose the conductors 96 may be run to one edge of the card on which'they are formed, and an elongated female receptacle shown in dotted outline at 102 may be provided to receive the edge of the card. The receptacle 102 contains separate contacts for engaging the conductors and connecting the same' to external circuitry As shown, two distributor cards are mounted in the. illustrated device; It is understood that the device may be modified to contain more of such circuit distributing members. In order to prevent yawing of the cards and; to assure positive alignment and engagement of the wiper arm contact buttons with the conductive areas on the face of each card with which it is associated, a strip 104 .of rigid material ispredisposed in the assembly operations along side of each spacer member 29 associating with the fastening bolts 22, 24, 26 and 28 and fitted in mating slits 106 in the frame members 10-10 and the distributor cards 94-94. Once the unitary assembly is bolted together a rigid structure results and the unit may then be removably pluggably engaged with one or more receptacles 102.

In order to connect the distributor cards 94-94 into one or more receptacles 102, the cards are extended slightly beyond'the edge of the frame members 10-10 as shown in Fig. l for convenient insertion into the recep tacles 102. In'order to accommodate a driving shaft and gear connection to the wheel 14, the distributor cards are offset to' one side of vertical plane of the coaxial shafts 72 and 73 and a suitable-notch or cut out 108 is provided in each of the frame members 10-10 above the shafts as shown in Fig. 1. In the plug-in position of the device, a driving shaft indicated-indotted outline at'110 extends through the cut outs 108 and is provided witha gear 11-2, also shown in dotted outline, which engages'the'gear wheel 14. The shaft 110 and gear 112 are connected to a source of constantly applied torque such as a' continuously operating electric motor (not shown).

Since the solenoid 30 does not of itself cause the switch to step, electrical current of low amperage, for example, approximately fifty milliamps, is required. This makes for much faster switch operation inasmuch as a relatively small solenoid coil having a low inductance can then be used which provides avery steep rate of current rise unlike the earlier-mentioned coil requiring one-half ampere. In addition, the control circuitry which is utilized to step the switch, pulse the electromagnet'o'r solenoid, et cetera, may thus be low powered. This tends to prevent the wear usually associated with pulsing contacts.

In operation, the program scanner is actuated inthe following manner:

Assume that power is applied from an'external driving mechanism through the gear 110 and that this rotative force is coupled to the drive gear 14 so'that this latter gear and the frictional coupling 16 are rotating together as a unit at a constant predetermined velocity. When the solenoid 30 is pulsed over the leads 34-34, it is energized and attracts the armature 48 and causes it torock the escapement arm 56 in a counter clockwise direction. This movement of the escapement arm causes the upper projection or pallet 68'of' the escapement arm to be withdrawn from between two adjacent teeths of the ratchet wheel 70. V I

At thesarne instant the lower- *projectiomor pal-let 66 moves in between two adjacent teeth of the wheel 70. Because of the constant torque being applied by the clutch arms 75-75 this rocking movement of the escapement arm permits the wheel 70 to move a slight amount but not suflicient to step the wheel one complete tooth. While the solenoid is energized the lower pallet 66 is maintained between the two teeth into which it has been inserted thus holding the wheel 70 from further rotation. At the instant the solenoid 30 is deenergized, i.e., the pulse stops, the torque applied through the friction coupling 16 is sufiicient to rotate the escapement wheel 70 and push the pallet 66 out of the way and at the same time bringing pallet 68 in between the next adjacent set of teeth to its previous position and thus prevent the ratchet wheel 79 from further turning. This action permits the ratchet wheel to advance or rotate a tooth at a time. In this fashion a novel reset mechanism is built into the escapement portion of the apparatus since the applied torque actually cams the escapement into a cocked or reset condition. Since the ratchet wheel is integral with the shaft to which the distributor wipers are fastened, the wipers, carrying their contact elements, are thus caused to automatically advance over the conductive areas 96 and 1% of the distributor mechanism, one area or lane at a time.

Past operation speeds are thus possible due to the design and the utilization of an electromagnetically controlled escapement mechanism as a triggering device rather than as an actuating or driving element. Pulses applied to the solenoid 30 are of short duration, such as a small fraction of a second, and since the function of the solenoid is not to drive the escapement wheel but merely to trigger its movement, very little power is needed for this function. A program scanning stepping switch fabricated according to the invention as herein described has been actuated at the rate of two hundred steps per second.

There has thus been described a novel electromechanical program scanning apparatus in the form of a rela tively high-speed stepping switch in which an escapement mechanism, which is cocked by the armature of a solenoid, is utilized to detent a ratchet wheel and which is subject to a constant torque applied through a friction coupling. Thus the ratchet wheel and distributor are capable of being driven at selected random intervals to thereby automatically step the device from one conductive lead to another.

What is claimed is:

1. An electromechanical stepping switch comprising, in combination, a rotatable shaft, a drive wheel fixed to said shaft'for joint rotation therewith, means for continuously rotating said drive wheel, an escapement wheel rotatably disposed on said shaft adjacent said drive wheel, a friction coupling secured to said drive wheel yieldingly gripping said escapement wheel and tending to rotate said escapement wheel, an electromagnetic device provided with a movable signal responsive element, an escapement arm engaged by said signal responsive element and movable in response to the movement of the element, said escapement arm having oppositely disposed projections thereon adapted to alternately engage the teeth of said escapement wheel so as to cause said escapement wheel to escape a tooth each time said electromagnetic device is energized.

2. An electromechanical stepping switch comprising, in combination, a rotatable shaft, a drive wheeel mounted on said shaft for rotation relative thereto, means for continuously rotating said drive wheel, an escapement wheel on said shaft and being .disposed adjacent to said drive wheel, a friction coupling secured to said drive wheel, said coupling being constantly urged into yieldingly gripping engagement with said escapement wheel and tending to drive said escapement wheel, an electromagnetic device provided with a movable signal responsive element, an escapement arm engaged by said signal responsive element and movable in response to the movement of the element, said escapement arm having integral oppositely displaced projections thereon adapted to alternately engage the teeth of said escapement wheel as the arm is moved so as to permit said escapement wheel to rotate a tooth each time the said electromagnetic device is energized.

3. An electromechanical stepping switch comprising a pair of supports, a shaft rotatably journaled between said supports, a toothed drive wheel secured to said shaft, means for rotating said drive wheel at a substantially constant velocity, a second shaft rotatably journaled on the first-mentioned shaft, an escapement wheel secured to said second shaft, a pair of jaw-like clutch arms hingedly mounted to said drive wheel and surrounding a portion of said escapement wheel, means yieldingly urging the arms into frictional gripping engagement with said escapement wheel thereby tending to transmit a constantly applied torque from said drive wheel to said second shaft, and electromagnet secured to one of said supports, said electromagnet being provided with a pivotally mounted armature, an escapement arm rockably mounted and having one end engaging the free end of said armature, the other end of said escapement arm having a pair of oppositely disposed pallets thereon, said escapement arm rocking in response to the rocking movement of the armature and arranged to have its pallets alternately engage the teeth of the escapement wheel in response to such rocking movement, said escapement wheel being movable under the influence of the constant torque applied through said clutch arms when the escapement arm is rocked, and a switch member secured to said second shaft and movable in stepped manner in timed relation to the movement of the escapement wheel.

4. A high speed electromechanical program scanner comprising side frames, a shaft journaled at opposite ends hereof between said frames, a toothed escapement wheel integral with said hollow shaft and being provided with an enlarged hub, a toothed drive wheel, means for continuously rotating said drive wheel at a constant high speed, said drive wheel being mounted for rotation concentric with and adjacent to said escapement wheel, two C-shaped clutch arms pivoted in jaw-like fashion to said drive wheel, said clutch arms frictionally surrounding and yieldingly gripping said enlarged hub so as to normally cause said escapement wheel and said shaft to rotate in unison, a signal responsive solenoid secured to one of said frames, said solenoid being provided with an armature, an escapement arm operably engaged by said armature and having two oppositely disposed camming projections at the free end thereof, one of said camming projections being urged by said armature into engagement with the teeth of said escapement wheel to as to prevent rotation of said escapement wheel in response to said applied torque, the other said camrning projections being maintained free of engagement with said toothed wheel until cammed into engagement therewith by movement of said armature in response to the energization of said solenoid, and a distributor mechanism provided with a plurality of conductors forming an electrical circuit pattern, wiping contactors carried by said shaft and engaging said conductors upon rotation thereof.

5. A high speed input-output device in the form of a program scanner adapted to distribute electrical potentials in a step-by-step sequence comprising a pair of parallel frames, a hollow shaft, a second shaft journaled between said frames, said hollow shaft being mounted for rotation about said second shaft, a ratchet wheel and an enlarged hub both integral with said hollow shaft, an escapement mechanism including an escapement arm pivotally mounted on one of said frames and rockably engaging the teeth of said ratchet wheel, a solenoid provided with a clapper also mounted on one of said frames adjacent said escapement mechanism, said escapement arm being engaged by said clapper so that upon rocking motion of said clapper said escapement is rocked into and out of engagement with the teeth of said ratchet wheel, a toothed wheel mounted on said second shaft concentric with said ratchet wheel, means driving said toothed wheel at a constant high speed, a pair of spring biased clutch arms, each arm being pivoted at one end thereof to said toothed wheel,the other ends of said arms being urged toward each other so as to yieldingly surround and grip said enlarged hub so that upon rotation of said toothed wheel said hub will be rotated in unison therewith, means mounting a plurality of rotary wipers to one end of said hollow shaft, each one of said wipers bearing an electrically conductive contact at the end thereof, and a plurality of: conductive elements engaged by the contacts of said wipers upon rotation of said hollow shaft.

6. A pluggable stepping switch comprising, a pair of frames, a plurality of electrically insulating members carried by said frames and disposed therebetween in spaced apart parallel relation to one another and the frames, said insulating members each being provided with a shaft re ceiving aperture aligning with those of the other members, individual conductors disposed in a radial pattern around a portion of said apertures on both sides of said insulating members, a common conductor disposed around another portion of said apertures on both sides of each of said insulating members, the individual and common conductors of one side being in registry with the individual and common conductors of the opposite side of each of said insulating members, a rotatable shaft extending through said aligned apertures, conductive wipers carried by said shaft and acting to cause said common conductors to successively contact each one of said individual conductors upon rotation of said shaft, said insulating members and said conductors extending in the same direction beyond a common edge each of said frames so as to be receivable within a mating receptacle, and a cover extending around the remaining edges of said frames and slidably receivable thereover to enclose portions of said insulating members lw'ng between said frames to thereby protect said members from the ingress of dust and other foreign material.

7. A pluggable stepping switch comprising, a pair of frames, a plurality of electrically insulating members carried by 'said frames and disposed therebetween in spaced apart parallel relation to one another and the frames; said insulating members each being provided witha shaft receiving aperture aligning with those of the other members, individual conductors disposed in a radial pattern around a portion of said apertures on both sides of said insulating members, a common conductor disposed around another portion of'said apertures on both sides of each of said insulating members, the individual and common conductors of one side being in registry with the individual and common conductors of the opposite side of each of said' insulating members, the common conductors on each side of each one of said insulating members being circularly displaced 180 with respect to the individual conductors on each side thereof, a rotatable shaft extending through said aligned apertures, conductive wipers carried by said shaft and acting to cause said common conductors to successively contact each one of said individual conductorsu'pon rotation of said shaft, said insulating members and said conductors extending in the same direction beyond a common edge of each of said frames so as to be receivable within a mating receptacle, and a cover extending around the remaining edges of said frames and slidably receivable thereover to enclose portions of said insulating members lying between said frames 'to thereby protect said members from the ingress of dust and other foreign material.

8.'A pluggable stepping switch comprising, a pair of frames, each one of said frames being provided with a separate notch in a common edge thereof, a plurality of electrically insulating members carried by said frames and disposed therebetween in spaced apart parallel relation to one another and the frames, said insulating members each being providedwith a shaft receiving aperture aligning with those of the other members, each one of said insulating members being provided with an under cut portion at one end thereof forming a notch aligning with the notches on the other insulating members for receiving a power shaft therein, individual conductors disposed in a radial pattern around a portion of said apertures on both sides of said insulating members, a common conductor disposed around another portion of said apertures on both sides of each of said insulating members, the individual and common conductors of one side being in registry with the individual and common conductors of the opposite. side of each of said insulating members, the common conductors on each side of each one of said insulating members being circularly displaced 180 with respect to the individual conductors on each side thereof, the individual conductors on each side of each of said insulating members being angularly displaced with respect to said notches so as to provide sufficient clearance between each one of said conductors and said shaft, a rotatable shaft extending through said aligned apertures, conductive wipers carried by said shaft and acting to cause said common conductors to successively contact each one of said individual conductors upon rotation of said shaft, said insulating members and said conductors extending in the same direction beyond a common edge of each of said frames so as to be receivable within a mating receptacle, and a cover extending around the remaining edges of said frames and slidably receivable thereover to enclose portions of said insulating members lying between said frames to thereby protect said members from the ingress of dust and other foreign material.

9. A pluggable stepping switch comprising, a pair of frames, each one of said frames being provided with a separate notch in a common edge thereof, a plurality of electrically insulating members carried by said frames and disposed therebetween in spaced apart parallel relation to one another and the frames, said insulating members each being provided with a shaft receiving aperture aligning with those of the other members, each one of said insulating members being provided with an under cut portion at one end thereof forming a notch aligning with the notches in the other insulating members for receiving a power shaft therein, individual conductors disposed in a spoke-like radiating pattern around a portion of the area defining each of said shaft receiving apertures on both sides of each one of said insulating members, each one of said individual conductors being provided with an integral extension disposed on said insulating members in parallel and spaced apart relation, each of said parallel extensions projecting beyond a common edge of each of said frames, a common conductor disposed around another portion of said apertures on both sides of each of said insulating members, the individual and common conductors of one side being in registry with the individual and common conductors of the opposite side of each of said insulating members, the common conductors on each side of each one of said insulating members being circularly displacedl with respect to the individual conductors on each side thereof, the individual conductors on each side of each of said insulating members being angularly displaced with respect to said notches so as to provide sufiicient clearance between each one of said conductors and said shaft, a rotatable shaft extending through said aligned apertures, conductive wipers carried by said shaft and acting to cause said common conductors to successively contact each one of said individual conductors upon rotation of said shaft, said insulating members and said conductors extending in the same direction beyond a common edge of each of said frames so as to protect said members from the ingress of dust and other foreign material.

10. A pluggable stepping switch comprising, a pair of frames, a plurality of electrically insulating members carried by said frames and disposed therebetween in spaced apart parallel relation to one another and the frames, said insulating members each being provided with a shaft receiving aperture aligning with those of the other members, individual conductors disposed in a radial pattern around a portion of said apertures on both sides of each of said insulating members, a common conductor disposed around another portion of said apertures on both sides of each of said insulating members, the individual and common conductors of one side being in registry with the individual and common conductors of the opposite side of each insulating member, a rotatable shaft extending through said aligned apertures, conductive wipers carried by said shaft and acting to cause said common conductor to successively contact each one of said individual conductors upon rotation of said shaft, each one of said Wipers being of S-shaped configuration and carrying a conductive contact at opposite ends thereof, the conductive contacts of said wipers being 180 displaced with respect to each other, said insulating members and said conductors extending in the same direction beyond a common edge of each of said frames so as to be receivable within a mating receptacle, and a cover extending around the remaining edges of said frames and slidably receivable thereover to enclose portions of said insulating members lying between said frames to thereby protect said members from ingress of dust and foreign material.

11. A step by step driving mechanism comprising, a first toothed driven member having a hub, a second toothed driving member, means rotating said second toothed member at a constant velocity, a pair of symmetrical clutch arms pivoted to said driving member, said clutch arms frictionally engaging said hub in tangential line contact therewith, an escapement contacting said first toothed member, and an electromagnet having a clapper operatively controlling the engagement of said escapement with said first toothed member whereby said first member is permitted to escape rotatively in step by step increments.

12. A step by step driving mechanism comprising, in combination, a first toothed driven member having a hub, a second toothed member, means rotating said second toothed member at a constant velocity, a pair of symmetrical clutch arms pivoted to said second member, said 10 clutch arms being frictionally engaged with said hub, an escapement contacting said first toothed member, and an electromagnet having a clapper operatively controlling the engagement of said escapement with said first toothed member whereby said first member is released for rota tion in step by step increments.

13. A step by step driving mechanism comprising, a first toothed driven member, a second toothed member, means for driving said second toothed member continuously, a pair of clutch arms pivoted to said second member, means frictionally biasing said clutch arms against the driven member, an escapement rockably contacting said first toothed member, and an electromagnet having a clapper operatively controlling the engagement of said escapement with said first toothed member whereby said first member is released for step by step rotation.

14. An electromechanical step by step drive mechanism comprising, in combination, a drive wheel, means for continuously rotating said drive wheel to an escapement wheel having electrical circuit interrupting means secured thereto disposed adjacent said drive wheel, clutch means anchored to said drive wheel, means biasing said clutch means into frictional engagement with said escapement wheel, an escapement rockably contacting said escapement wheel, and an electromagnet operatively associated with said escapement so as to control the engagement of said escapement with said escapement wheel whereby said escapement wheel is released to rotate in step by step increment so as to cause said circuit interrupting means to open and close electrical circuits.

References Cited in the file of this patent UNITED STATES PATENTS 291,705 Field Jan. 8, 1884 1,001,091 Thomassin Aug. 22, 1911 1,631,982 Potts June 14, 1927 2,171,044 Nelsen et al Aug. 29, 1939 2,531,882 Johnson et al. Nov. 28, 1950 2,559,075 Horlacher July 3, 1951 2,616,994 Luhn Nov. 4, 1952 2,649,513 Luhn Aug. 18, 1953 2,689,281 Hooker Sept. 14, 1954 2,698,884 Breitenstein Jan. 4, 1955 2,710,896 Graybill et al June 14, 1955 2,739,485 Sengebusch Mar. 27, 1956 FOREIGN PATENTS 658,244 Germany Mar. 28, 1938 

